This invention relates to a process for printing on a surface, and more particularly, to the process for printing data on a surface with high resolution by means of electrophotography.
Generally, the electrophotographic technique forms an electrostatic counterpart of an optical image on a photosensitive drum. The electrostatic latent image counterpart selectively attracts a toner material which is then transferred to a sheet of paper. The toner is heat-fused to bond it to the paper.
One popular technique for forming the electrostatic latent image employs a scanned laser beam (Light Amplification by Stimulated Emission of Radiation) for discharging portions of a charged photosensitive drum. The discharged portions correspond to those areas in which printing is not desired. The character or picture images are first stored in a memory. The elements stored in memory are then used to control the pulse of the laser as it scans the photosensitive surface.
In addition to printing on sheets of paper, it would be desirable to use such electrophotography to print on the surfaces of objects such as, for example, beer or soft drink cans, food storage containers and food storage pouches. To do this, stored image data is copied from memory to a photosensitive drum from which it is printed directly on the surface of the object. The electrophotographic method provides not only black and white printing but also color printing. For color printing, color separations of the image are printed onto the object using different color toners during separate applications. When the image is printed directly on the object in this way, the need for affixing a label to the surface of the object is eliminated.
Furthermore, since image data is stored in memory, it is easy to correct and change the image. This flexibility offers high efficiency compared to prior art printing methods.
In the prior art, printing characters with a laser printer, a resolution of from 240 to 400 dots per inch (DPI) horizontally and vertically is attainable. In computer color display applications, the apparent optical power can be improved by controlling color tones in each dot. Common computer graphics applications, together with suitable video controllers and color displays, permit display of up to 256 tones.
The same control of color tone is not available when printing on an object as when displaying color on a computer display. One of the reasons for this difference is that deposition of the toner is not precise, even though the electrostatic latent image can be formed with enough resolution. The electrostatic latent image is produced using pulse width modulation of the laser beam to discharge each dot according to the image tone. If the diameter of the toner fraction is not exactly matched to the tone of the electrostatic latent image, multiple tone images can not be represented with sufficient accuracy.
For surface printing, it has been found that pulse width modulation of multiple tones on each dot suffers a serious drawback imposed by the speed at which the images must be exposed on the photosensitive surface. This drawback is caused by the high throughput of the product being printed. For example, in commercial operation, it is desirable to print the surfaces of drink cans at the rate of 200 cans per minute. This requires a scanning velocity of the laser beam sufficient to expose about 20 mega dots per second. It is impossible to accomplish pulse width modulation for 256 tones during the 50 nanosecond period available to form each dot.
Continuous printing is necessary to maintain production rates. The foregoing drawbacks require careful thought to improving the technique for exposure with a laser beam.